Microscopes are frequently used for detecting small structures which cannot be made out with the naked eye and for discovering characteristic features in such structures. A fundamental use for microscopy in cytology, histology and pathology is for reviewing a specimen and looking for structures, cells or combinations of cells and the like which are of interest. If the sites of these structures are found on the specimen, it is desirable to note them, for a variety of reasons. For example, the structure found has to be located again at a later stage by the same or a different user for checking, for further inspection or for quality control. For this purpose, many microscopes have a unit for determining the coordinates of positions of a point in an instrument-dependent system of coordinates. By electromechanically determining these coordinates, it is possible to return to the location discovered at a later stage.
As a rule, however, the coordinates are instrument-dependent, i.e. the coordinates for this instrument can only be accurately reproduced if no changes have been made to the adjustment of the microscope and no tolerances were present. However, if for example the microscope stage is taken away for repair and reinstalled, it will give different coordinates for the same location on the specimen from those originally determined. Moreover, the coordinate systems of different microscopes, even of the same type, are not (exactly) the same.